For optical materials, in particular, for spectacle lenses, a method of using an episulfide compound is known from the viewpoint of physical properties (Patent Document 1). Further, as a method for further improving physical properties, for example, a composition in which a thiol compound is added to an episulfide compound for improving weather resistance (Patent Document 2) and a composition which consists of sulfur, episulfide and thiol for providing a higher refractive index (Patent Documents 3 and 4) have been proposed.
However, it is difficult to store such a thiol compound for a long period of time because of its high reactivity, and the purity is further reduced during long-term storage. There is a problem that white turbidity is caused in a composition containing such a thiol with a reduced purity when it is polymerized and cured. Because of intended use for optical materials, if white turbidity is caused after curing, all become defective products, resulting in a great loss. Accordingly, a technique of obtaining an optical material having excellent weather resistance, wherein the purity is not reduced during long-term storage, has been desired.